Apparatus for controlling oscillator grid drive



Oct. 2, 1956 R. R. MOORE 2,765,388

APPARATUS FOR CONTROLLING OSCILLATOR GRID DRIVE Filed March 30, 1953 F 2 F 3 x 0 lg. g 30 3| 34 33 0 .t E 8 Height Max. Height Max.

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Height Max.

2,765,388 Patented Oct. 2, 1956 2,765,388 APPARATUS FOR CONTROLLING OSCILLATOR GRID DRIVE Richard R. Moore, Louisville, Ky., assignor, by mesne assignments, to National Cylinder Gas Company, Chicago, 11]., a corporation of Delaware Application March 30, 1953, Serial No. 345,663 11 Claims. (Cl. 219-10.55)

This invention relates to high-frequency dielectric heating systems in which an oscillator supplies high-frequency power to a dielectric load disposed between spaced heating electrodes and particularly relates to arrangements for maintaining the grid-excitation of the oscillator substantially constant for different spacings of the heating electrodes.

In certain types of dielectric heating systems including some disclosed in copending U. S. application, Warren, Serial No. 138,628, filed January 14, 1950, now abandoned in favor of continuation-in-part application, Warren, Serial No. 419,633, filed March 26, 1954, the gridexcitation voltage for the oscillator is derived from the potential-difference between the heating electrodes. in such systems, as the electrode spacing is changed to accommodate the physical or electrical characteristics of different loads, the electrode voltage changes with consequent change of grid-excitation. The grid-excitation voltage may either increase or decrease with decreased electrode spacing depending upon the particular oscillator circuit with the result that the oscillator tube may operate inefficiently, and it may be damaged or destroyed by excessive grid or anode dissipation.

In accordance with the present invention, a reactancc for coupling the oscillator grid to the heating electrode is adjusted automatically upon change in electrode spacing in sense and to extent to compensate for the effect upon the grid-excitation of the change in electrode voltages. More specifically, the grid-coupling reactance comprises a capacitor having a plate movable with the movable heating electrode and a second plate adjacent the path of movement of the movable plate. The reactance of said grid-coupling capacitor, throughout its range of adjustment with the heating electrode, is high with respect to the reactance of the effective input capacitance of the oscillator tube which varies automatically with plate load of the oscillator so that the grid-excitation is automatically compensated for changes in interelectrode impedance due to changes in electrode spacing, or change in properties of the load, or both.

For a more detailed understanding of the invention and for an illustration of a preferred form thereof, reference may be had to the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 diagrammatically illustrates a dielectric heating system embodying the present invention; and

Figs. 2, 3 and 4 are explanatory figures referred to in general discussion of the electrical operating characteristics of the system illustrated in Fig. 1.

Referring to Fig. 1 of the drawings, the invention in one form is shown applied to a high-frequency dielectric heating system including a high-frequency heating applicator comprised of electrically conductive walls 1114 and having mounted therein elements providing substantially all the inductance and capacitance of the applicator. Although the applicator may be completely open at both ends, it is preferably a more complete enclosure to mini-- mize leakage of the intense magnetic and electrical fields existing therein.

Substantially all the inductance for the applicator may be provided by an electrically conductive structure or fin 15 which, in this particular embodiment, is shown secured to one of the conductive walls of the applicator, usually the top wall 11, and as shown depends therefrom spaced from all the other walls. The structure or fin 15 illustrated is of the type disclosed in Ellsworth Patent 2,711,468, dated June 21, 1955, for Dielectric Heating Tunnels. It is to be understood, however, that other fin structures may be used satisfactorily, such as for example, those disclosed in the aforesaid Warren applications and also Warren applications Serial Nos. 419,070 and 419,071, both filed March 26, 1954, as continuations-in-part of the aforesaid application Serial No. 138,628. The length of the structure 15 from the attached end to the depending end is small compared to a quarter-wavelength at the excitation frequency of the applicator. The structure 15 may be considered as a lumped inductance in which the current gradient is negligible.

The heating of a load 16, such as plastic preforms, subjected to the action of the applicator is produced by a high-frequency electric field generated between an upper heating electrode 17 and the wall 13 which may serve as the lower heating electrode. Although not specifically disclosed in Fig. 1, a metallic conveyor may serve as the lower heating electrode so as to provide for the continuous processing of work. The upper electrode 17, which may consist of a plate of electrically conductive material such as aluminum and preferably rectangular in shape, though not necessarily so, is secured or otherwise electrically connected to the free end of the fin structure 15 of the applicator, usually at a portion intermediate the sides and ends thereof.

The distance from any point of the junction between the structure 15 and electrode 17 to the nearest point of an edge or an end of the electrode is usually and prefer ably small compared to a quarter-wavelength at the ex citation frequency. The capacity between the electrodes of the applicator may be treated as a lump capacitance. Usually and preferably the length of electrode 17 projecting beyond the fin 15 is such that all points on the electrode will be substantially of the same uniform voltage.

In the system shown in Fig. 1, the applicator 3.0 is ex cited by a coupling loop 18 which may be a wide band of copper fastened to the outside of a form provided by copper piping which also serves to conduct cooling liquid for the anode of the oscillator tube. Such an arrangement is disclosed and claimed in copending Warren application Serial No. 419,074 which is a continuation-in-part of said Warren application Serial No. 138,628.

The electrode 17 may be raised or lowered by means of a lifting apparatus represented by struts 17a which are mechanically connected at one end to the electrode 17 and pass through the top Wall 11 of the applicator 10. The bottom portion of the fin 15 is provided with accordionlike metallic pleats which, together with the solid walled portions of the fin structure, at all times maintain a complete shielding of the lifting apparatus from the high-frequency magnetic field circulating about the fin 15 above the electrode 17. The foregoing features are disclosed and claimed in copending Warren applications Serial Nos. 419,633 and 419,070.

Many various types of oscillator circuits may be used for exciting the applicator herein shown. The preferred oscillator 2i) illustrated is of type described and claimed in the aforesaid Warren applications Serial Nos. 138,628 and 419,633.

In the oscillator system 20, the coupling loop 18 of the tunnel or applicator 10 is included in the plate or anode circuit of an oscillator tube 21 by conductor 22. The grid of tube 21 is connected to the fin and electrode structure 17 through connection to the heating electrode 17 by way of a lead 23 passing through an insulator in the side wall 12 of the applicator 1i and a reactance, illustrated as a capacitor 24, which, in accordance with the present invention, is disposed within the applicator. The cathode of tube 21, so far as the operating frequency of the tunnel is concerned, is grounded through the bypass capacitors 25. A direct-current source of high voltage 13+, B is connected between ground and the cathode of the tube 21, the positive side of that source being grounded as indicated. A direct-current path between the grid and cathode of tube 21 is provided by radio-frequency choke 26 and grid leak resistor 27.

A capacitor 28, in whole or for the most part provided by the etlective input capacitance of tube 21, is in series with the capacitor 24 to provide a capacitive potential divider between the electrodes 17, 13. This potential divider, as described and generically claimed in the aforesaid Warren application Serial No. 419,633, operates, because of the automatic variation of capacitor 28, to effect automatic stabilization of the grid voltage by compensating, at least in part, for the effect of changes in power factor of the applicator which may result from changes in characteristics of the load being treated or from variations in spacing between the heating electrodes. Variations in grid voltage may result not only from such changes in power factor of the applicator, but also from variations in voltage betwen the heating electrodes as a result of variation in the spacing between the heating electrodes.

In accordance with the present invention, the capacitor 24 is automatically adjusted, upon change in spacing between electrodes 17, 13, in compensation for the change in electrode voltage due to the spacing changes etfected to accommodate different work. In the arrangement shown, the voltage of the electrode 17 will increase as the space between it and the lower electrode or wall 13 is decreased and will decrease as the space is increased. This relationship is illustrated in Fig. 2 by the line 30. It is possible to have arrangements whereby an opposite condition will prevail: that is, where the electrode voltage increases as the electrode 17 is raised and decreases as the electrode is lowered. The latter relationship is illustrated in Fig. 2 by the line 31.

Assuming now for the purposes of general discussion that the former conditions apply, it will be observed that if the capacitance of the grid-coupling capacitor 24 were to remain constant for different spacings between electrodes 17, 13, the voltage applied to the grid of the tube 21 will increase as the electrode 17 is lowered and will decrease as the electrode is raised.

In said Warren application Serial No. 419,633 a grid coupling capacitor is located outside of the heating applicator. A conductor extends through a wall of the applicator and is connected to the movable heating electrode. Such a direct electrical and conductive connection with the electrode 17 introduces the possibility of failure by breakage of a connecting Wire and also limits the movement of the electrode to the length of the connecting wire. To avoid these limitations and in order automatically to compensate for the variations in grid volta e introduced by changes in the electrode spacing, the grid-coupling capacitor 24 is here placed Within the chamber of applicator 10 and is comprised of a plate 24a which is fixedly mounted preferably to an edge of the electrode 17 and disposed upwardly for movement relative to a second normally stationary plate 2411. Plate 24b is mounted on a shaft 32 extending through the wall 12 of the applicator and adapted for movement by a knob 32a toward and away from the associated plate 24a in adjustment of the maximum capacity of the coupling condenser 24.

When the upper heating electrode 17 is in its lowermost position, as illustrated by the dotted lines (Fig. l), the electrode voltage is at a maximum (curve 39, Fig. 2) and the capacitance of the grid-coupling capacitor 24 is at a minimum. This latter condition is illustrated in Fig. 3 in which line 33 represents the variation in capacitance of coupling capacitor 24 as the height of the electrode 17 is varied. The component of grid drive derived from the electrode voltage is dependent upon the capacitance of the capacitor 24. As the heating electrode 17 is raised, its voltage decreases and simultaneously therewith there is an increase in capacity of capacitor 24, resulting in a larger percentage of electrode voltage being placed upon the grid of the tube 21. The overall effect of the variations in electrode voltage and in the value of the capacitor results in a stabilization of the grid-excitation or drive to provide a substantially constant value of gridexcitation despite changes in electrode height. This desired condition of stabilized grid-excitation is illustrated in Fig. 4.

The plates 24a and 24b may be of equal area, but not necessarily so, and although illustrated to be of rectangular configuration may be of any desired shape. In setting up the dielectric heating system for operation on any given plastic preform or article to be treated, the capacitor plate 2412 may be moved relative to its associated plate 24. 1 by way of the shaft 32 and a knob 32a mounted thereon in order initially to set the value of the capacitor 24 to obtain predetermined amplitude of grid drive or excitation. Once the capacitor plate 24b has been set, nothing more is required of an operator inasmuch as potential variations in grid-excitation normally introduced by variations in electrode spacing will be compensated for by changes in capacitance of capacitor 24 due to the movement of plate 24a relative to plate 24b.

' In order that capacitor 24 shall also effectively serve as an element of a potential-divider compensating for change in power-factor of work being heated, the reactance of capacitor 24 at its maximum value of capacitance should be high compared to the effective reactance of the input capacitance of tube 21.

Where the conditions of operation are such that the electrode voltage decreases as the spacing between electrodes is narrowed (line 31 of Fig. 2), the capacitor 24 should be modified to have a characteristic such as illustrated by line 34 of Fig. 3. This characteristic may be etfected by so relocating the position of the capacitor plate 24a on the fin-electrode structure 15, 17 relative to the plate 241) that instead of having a minimum value of capacitance when the electrode 17 is in its lowermost position (dotted lines Fig. l), the capacitor 24 will then have a maximum value of capacitance. This change may also be brought about by relocating the position of the capacitor plate 24b and shaft 32 on the wall 12 and relative to the plate 24a, or by having plate 24b unsymmetrically mounted on shaft 32 for rotation to effect maximum capacity of capacitor 24 for either the upper or the lower position of electrode 177 It is apparent from the foregoing disclosure that the capacitor 24 serves two electrical purposes in the system of Fig. 1. It cooperates with the effective input capacitance of tube 21 (as represented by capacitor 28) to stabilize the grid-excitation against variations in powerfactor of the load being treated in the high-frequency heating applicator, and also stabilizes the grid-excitation against changes in electrode voltage introduced by variations in electrode height. It may be used to advantage in other oscillator systems to obtain only the last-named result.

What is claimed is:

l. A dielectric heating system comprising an oscillator tank circuit including heating electrodes variably spaced to accommodate different loads to be heated, and a grid feedback capacitor for an oscillator tube, said capacitor comprising a stationary plate for connection to a grid of said tube and a plate electrically connected to and movgreases able with one of said heating electrodes; the capacitance between said plates varying upon change in spacing of said electrodes to maintain the grid drive'of said tube substantially constant.

2. A dielectric heating system comprising an oscillator tank circuit including heating electrodes variably spaced to accommodate different loads to be heated, and a grid feedback capacitor for an oscillator tube, said capacitor comprising a stationary plate for connection to a grid of said tube and a plate electrically connected to and movable with one of said heating electrodes, the capacitance between said plates varying upon change in spacing of said electrodes in sense opposite to the change in spacing between said electrodes so to maintain the grid drive of said oscillator tube substantially constant.

3. A dielectric heating system comprising an oscillator tank circuit including heating electrodes variably spaced to accommodate different loads to be heated, and a grid feedback capacitor for an oscillator tube, said capacitor comprising a stationary plate for connection to a grid of: said tube and a plate electrically connected to and movable with one of said heating electrodes, the capacitance between said plates varying upon change in spacing of said electrodes in the same sense as the change in spacing between said electrodes so to maintain the grid drive of said oscillator tube substantially constant.

4. A dielectric heating system having an oscillator tank circuit including heating electrodes variably spaced to accommodate different loads to be heated, means for maintaining the grid drive of an oscillator tube feeding the tank circuit substantially constant, said means comprising a grid feedback capacitor for the oscillator tube including a stationary plate and a plate electrically connected to and movable with one of said heating electrodes, said capacitor forming with the grid to cathode capacitance of said oscillator tube, a capacitive voltage divider to cornpensate for variations in power factor of a load being treated by the system, the capacitance between said plates varying upon change in spacing of said electrodes to vary the fraction of electrode voltage applied as grid device.

5. In a dielectric heating system having an oscillator tube, an oscillator tank circuit including heating electrodes variably spaced to accommodate different loads to be treated, said oscillator tube deriving excitation voltage from one of the heating electrodes, reactance means for maintaining the excitation voltage substantially constant despite variations in amplitude of the electrode voltage produced by changes in spacing between the electrodes, said means comprising in series with the input capacitance of said tube, a grid feedback capacitor having a stationary plate and a conductive plate electrically connected to and movable with one of said heating electrodes, the capacitance between said plates varying upon change in spacing of said electrodes to vary the fraction of electrode voltage selected for excitation of said oscillator tube.

6. A dielectric heating system in accordance with claim 5 in which the relatively stationary plate is fixed against movement in a plane defined by the plate, and means for -moving said plate toward and away from the plate mounted on one of the heating electrodes to vary the capacitance thereof.

7. A dielectric heating system comprising an enclosure having metallic walls, an oscillator tank circuit including heating electrodes variably spaced to accommodate dif fercnt loads to be heated and positioned within said enclosure, an oscillator tube, and a grid feedback capacitor for the oscillator in series with the effective grid-cathode capacitance of said tube comprising a plate mounted on one of the walls of said enclosure and another plate electrically connected to and movable with one of said heating electrodes, said second-named plate being movable with said electrode in a direction to vary the capacitance between said plates by change in area between the plates and in accordance with change in spacing of said 6 electrodes to maintain the grid drive of said oscillator substantially constant.

8. A dielectric heating system comprising an applicator having heating electrodes variably spaced to accommodate different loads, an oscillator tube for supplying highfrequency power to said applicator, and means for deriving the grid-excitation for said oscillator tube from the potential difference between said electrodes comprising a variable grid-coupling capacitor mechanically connected for adjustment concurrently with change in spacing of said heating electrodes and electrically connected between said heating electrodes in series with the effective input capacitance of said tube, which latter electrically varies with change in power factor of the load.

9. In a system for high-frequency electric heating of dielectric material, a resonant applicator including electrodes supported in spaced relationship for receiving such material therebetween and inductance structure electrically connected at one end to, and extending away from, one of said electrodes, said applicator also including conductive structure electrically interconnecting the other end of said inductance structure with the other of said electrodes, said inductance structure cooperating with capacity-rneans including said electrodes to provide a resonant circuit, a power oscillator of which said resonant applicator forms a part and including an oscillator tube having effective input capacitance which varies with variation in loading between said electrodes, means for deriving from said applicator a potential which varies in accordance with variation in voltage between said electrodes, and a potential-divider upon which said derived potential is impressed, which potential-divider includes capacitance means in series with said effective input capacitance and cooperative therewith to provide across said effective input capacity a grid-excitation voltage which varies in cornpensatory sense with such variation in loading, said capacitance means comprising a stationary plate and a movable plate electrically connected to and movable with one of said electrodes, the capacitance between said plates varying upon change in spacing of said electrodes to vary the fraction of electrode voltage selected for excitation of the oscillator.

10. In a system comprising an oscillator tube for generating radio-frequency power for dielectrically heating work, a metallic reentrant cavity having metallic fin structure internally extending from wall structure of said cavity and an internal electrode structure electrically connected to said fin structure in spaced relation to said wall structure, said fin structure being extensible to vary the capacity of said internal electrode structure to accommodate different work-loads, a loop in the anode circuit of said tube and disposed in the cavity space which encircles said fin structure for transfer from said tube to said work of radio-frequency power, and a potentialdivider comprising capacitance electrically connected from said cavity-electrode structure to the grid of said tube and capacitance between the grid and cathode of said tube, said first-mentioned capacitance comprising a stationary plate and a plate electrically connected to and movable with said cavity-electrode structure, the capacity between said plates varying upon change in spacing of said electrodes to vary the fraction of voltage selected for excitation of said oscillator, said potential divider automatically regulating the voltage applied to said oscillator to maintain substantial constancy of the grid-excitation despite substantial variations in power-factor of the work.

11. In a dielectric heating system comprising spaced heating electrode structures adapted to accommodate th'erebetween dielectric material to be heated by an electric field between said structures, high-frequency power generating means connected to deliver high-frequency power to said electrode structures and including on oscillator circuit having an oscillator tube therein, said tube having a grid circuit including in series therein the interelectrode capacitance between its grid and cathode, means including the capacitance between said electrode structu-res providing a resonant circuit which predominantly determines the frequency of said oscillator, at least one of said electrode structures being movable relative to the other for variation of the spacing and of the electrode voltage between said structures, means for impressing upon the grid circuit of said oscillator tube a voltage derived from said electrode voltage so that said grid voltage tends to vary with variation in spacing between said electrode structures, said last-mentioned means including a variable capacitor comprising a pair of plates insulated from one of said electrode structures and connected in series in said grid circuit and between the other of said electrode structures and the grid of said tube, at least one of which plates is movable relative to the other for variation in the capacitance between said plates, and means operative to effect relative movement between said plates in response to relative movement between said electrode 8 structures to vary the capacitance between said plates in a sense to-counteract said tendency of said grid voltage to vary with variation in the spacing between said electrode structures.

References Cited in the file of this patent UNITED STATES PATENTS 2,124,029 Conklin et a1 July 19, 1938 2,197,124 Conklin Apr. 16, 1940 2,262,020 Llewellyn Nov. 11, 1941 2,467,782 Schman Apr. 19, 1949 2,504,109 Dakin et al. Apr. 18, 1950 2,666,129 Ellsworth Jan. 12, 1954 FOREIGN PATENTS 556,292 Great Britain Sept. 28, 1943 

